Help with shrinking my bottles?

[Spaceman Spiff]

Hi guys,

I'm trying to build a scale replica of the ARES-1 rocket in water rocket form, and I've found that I need to shrink the diameter of my bottles to get the right diameter for the SRB motor stage. I found that a metal tennis-ball can makes a great form to shrink over and it's within a couple percent of the exact diameter I need.

So far I have been able to make some really perfect rounded ends for butt splicing. I want the body to be smooth, so I don't want to overlap the joints.

The problem I get is when I try to shrink my skin sections then they always don't shrink perfectly. I get one or two spots where the bottles shrink less and it makes a wrinkle or warp in the outside of the skin. I am using pepsi bottles which only need to shrink about 1/2" so I don't think I'm trying to shrink them excessivly. They just come out crooked or not smooth on the outside.

I hope you guys can offer me some suggestions.

[The Mooseheads]

Hi guys,

I'm trying to build a scale replica of the ARES-1 rocket in water rocket form, and I've found that I need to shrink the diameter of my bottles to get the right diameter for the SRB motor stage. I found that a metal tennis-ball can makes a great form to shrink over and it's within a couple percent of the exact diameter I need.

So far I have been able to make some really perfect rounded ends for butt splicing. I want the body to be smooth, so I don't want to overlap the joints.

The problem I get is when I try to shrink my skin sections then they always don't shrink perfectly. I get one or two spots where the bottles shrink less and it makes a wrinkle or warp in the outside of the skin. I am using pepsi bottles which only need to shrink about 1/2" so I don't think I'm trying to shrink them excessivly. They just come out crooked or not smooth on the outside.

I hope you guys can offer me some suggestions.

Spaceman Spiff,

I suggest you try reconducting your experiment in shrinking by pre-heating an over to 450 degrees (not critical. Anything over 400 should work). Make sure your form (the metal can) is at room temperature. You want to have the bottle cool when it contacts the can so it does not heat anneal and become brittle. Use a stick to insert the form with bottle on it in the oven. I have made a opening in the door of my oven for this purpose so the heat does not escape. Rotate the stick by hand to prevent sagging from gravity. Remove after 20 seconds (PET takes 6-8 seconds to completely shrink).

Also, PET shrinking varies from one brand to another, so you should try different brands. Also, PET does not shrink the same amount lengthwise and widthwise. It typically shrinks 6:5 lengthwise and 6:5.5 by width.

Good luck!

[Spaceman Spiff]

Hi Rick,

Thanks for your insights. I have a couple of suggestions from several people and I will try them out.

I happened to find that Dr. Pepper bottles are slightly smaller in diameter and a lot closer to my target size, so I will problably just use these as they are without shrinking.

I still want to master the shrinking skill so I will try all the suggestions and see if I can't learn how to do it well.

Thanks again!

[The Mooseheads]

No problemo Spaceman!

Glad to help you out. Just keep us informed with the techniques you find out and let us know how that rocket is coming along!

[Spaceman Spiff]

Hi guys. I've been busy and didn't make much progress. I'm going to set aside some time to try all the suggestions. Meanwhile, I've got some photos of the crappy shrink jobs I've done so far.

[Spaceman Spiff]

Oops. It helps to add the photos before sending the post.

[The Mooseheads]

Long time no see, spaceman! Glad you're back at it! Let me know how you make out with the shrinking technique. It would be great to build our base of experience here so the new forum members can learn from our mistakes. Heck, one of these days I think I'm going to compile a F.A.Q. about water rockets and how to make them.

Hey David or Lisa, how do you feel about hosting a water rocket FAQ page on the site? I bet that many people would give input and make use of something like that!